It put the green in paint, wallpapers and food, people have used it since the Bronze Age, and it is part of the natural environment. Enormous tracts of America have groundwater polluted by arsenic. People in the Antofagusta region of northern Chile have been drinking water with a tinge of arsenic for 10,000 years. They die more frequently than their neighbours from skin, lung and bladder cancers.

Antofagusta is at least six times the size of Bangladesh, where many of the country's 10m tube wells sunk in the last 30 years draw water from geological strata rich in natural arsenic compounds. The World Health Organisation thinks 77 million Bangladeshis are at risk from arsenic poisoning, and calls it "the largest mass poisoning of a population in history".

It began with the best intentions: water-borne diseases killed 250,000 Bangladeshis each year in 1970, when aid agencies began to supply hand-pumped tube wells to deliver pure, pathogen-free water from ancient buried sediments. Andrew Meharg, a biogeochemist at the University of Aberdeen, is good on the technological and political challenges of testing water for safe levels, when it isn't too clear what a safe level would be. He is terrific on the wider history of arsenic, in alchemy, industry and interior decorating: 100m of wallpaper - enough to paper a large room - could contain 2.5kg of arsenic. In 1858, British homes contained 250m square kilometres of arsenic wallpaper, causing nationwide poisoning.

In Bangladesh, a WHO survey found that most residents did not know of the health effects of arsenic in their drinking water. It also found that 42% used contaminated tube wells because there was no other water. There are 80,000 villages in Bangladesh, a nation savaged by floods, drought and diarrhoea. What would be the cost of safe water for all of them? And what would be the cost of not providing it?

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